1. Field of the Invention
The present invention relates to a side-tunnel-type polarization-maintaining optical fiber, a method of manufacturing a polarization-maintaining optical-fiber connecting portion, and a polarization-maintaining optical-fiber connecting portion.
2. Description of the Related Art
The polarization-maintaining optical fiber is an optical fiber that propagates a light while maintaining a polarization state of the light. A stress-applying-type polarization-maintaining optical fiber in which orthogonal axes having different effective refractive index from each other are formed by arranging stress applying members in opposite to each other across the core region and an elliptic-core-type polarization-maintaining optical fiber including an elliptic core region are examples of the polarization-maintaining optical fiber.
On the other hand, a side-tunnel-type polarization-maintaining optical fiber has been researched since a long time ago as a polarization-maintaining optical fiber that does not require a stress applying member, and it is known that the single polarization characteristic can be obtained by adjusting the refractive index profile of the core region. For example, a polarization-maintaining optical fiber disclosed in Japanese Patent Application Laid-Open No. 2004-264405 is characterized in that a transmission loss caused by an OH loss and a leakage loss is reduced by taking a structure in which a gap is provided between the core region and holes.
However, the polarization-maintaining optical fiber disclosed in Japanese Patent Application Laid-Open No. 2004-264405 has a problem that the gap between the core region and the holes decreases the modal birefringence, so that the polarization maintaining property is degraded. Although it is described in Japanese Patent Application Laid-Open No. 2004-264405 that a modal birefringence equivalent to that of a standard stress-applying-type polarization-maintaining optical fiber can be obtained by considerably increasing the refractive index of the core region or by decreasing the core diameter, if the refractive index of the core region is increased, the nonlinear characteristic is increased because the mode field diameter is decreased. In addition, there is a problem that the macro-bending loss is greatly increased, if the core diameter is decreased.